1. Field of the Invention
The present invention relates in general to a PWM (pulse width modulation) switching regulator which is capable of changing an internal oscillation frequency into a known oscillation frequency in accordance with load condition to reduce power consumption and to enhance efficiency.
2. Description of the Related Art
A conventional PWM switching regulator circuit is usually operated at a fixed internal oscillation frequency irrespective of load condition. For example, there is known a control circuit for use in a PWM switching regulator shown in a circuit diagram of FIG. 4 (for example, refer to JP 11-155281 A (page. 4 and FIG. 1)).
The operation of the conventional PWM switching regulator controlling circuit will hereinbelow be described with reference to FIG. 4. An error amplifying circuit 102 amplifies a signal exhibiting a difference between a voltage signal Va which has been obtained by dividing a detected voltage signal Vout at an output terminal with bleeder resistors 100 and 101 and a voltage signal Vref outputted from a reference voltage circuit 106 to output the resultant signal in the form of an output voltage signal Verr. Then, a comparison circuit 103 compares a triangular wave Vosc outputted from an oscillator 104 and the level Verr of the output signal with each other to output the resultant signal in the form of a pulse signal EXT used to control an external switch through a buffer 105 to thereby carry out the control in such a way that the terminal voltage Vout becomes a desired constant output voltage. The outputted pulse signal EXT shows an ON/OFF state at the same cycle as that of an internal oscillation frequency Vosc because of the fixed internal oscillation frequency Vosc.
As described above, in an example of the circuit of the PWM switching regulator, since the internal oscillation frequency is usually fixed, the buffer is also usually operated at the same frequency as the internal oscillation frequency, and the external switch is also operated at the same frequency as the internal oscillation frequency to thereby carry out the control in such a way that the output voltage Vout becomes the desired constant voltage. In addition, since the PWM switching regulator is operated so as to hold the output voltage constant by repeatedly turning ON/OFF of the external switch, the noise having the same frequency as the oscillation frequency is contained in the output voltage.
However, since the PWM switching regulator usually oscillates at the same oscillation frequency, a filter for removing the noise having the oscillation frequency is provided in the output voltage line to allow the noise to be readily removed so that the stable power source can be readily obtained.
However, in the conventional PWM switching regulator, the oscillation circuit, the buffer and the external switch are operated at the fixed frequency irrespective of the condition of a load connected to the output terminal. For this reason, there is encountered a problem in that if the load connected to the output terminal supplying the voltage Vout becomes light, then a quantity of power consumed in the PWM switching regulator controlling circuit becomes more than that consumed in the load, which results in the remarkably deteriorated efficiency. The circuits which mainly consume the power in the PWM switching regulator controlling circuit are the internal oscillation circuit and the buffer which is operated at the same frequency as the internal oscillation frequency.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problem associated with the prior art, and it is, therefore, an object of the present invention to provide a PWM switching regulator with which an internal oscillation frequency of a PWM switching regulator controlling circuit is halved while the switching regulator is operated under a light load to nearly halve the power consumption of the PWM switching regulator controlling circuit to thereby obtain the high efficiency even while the switching regulator is operated under a light load. Thus, the feature of the present invention is that even the respect, as the advantage of a PWM switching regulator, that the noise contained in the output voltage is readily removed is not injured since an oscillation frequency is held at a known value.
A PWM switching regulator controlling circuit according to the invention of the present application includes: an oscillation circuit for generating an oscillation signal for a PWM switching regulator; a first pulse generator for receiving as its input the signal from the oscillation circuit to output a signal PULSE-MIN; and a second pulse generator for receiving as its input the signal from the oscillation circuit to output a signal PULSE-MAX. In addition, the PWM switching regulator controlling circuit further includes: a comparison circuit for comparing the signal from the oscillation circuit with a reference signal from a reference voltage circuit to output a signal Vcomp used to control an external switch; and a logical circuit for receiving as its input the signal PULSE-MIN, the signal PULSE-MAX, and the signal Vcomp to output a signal used to control an oscillation frequency of the oscillation circuit.
Further, in the PWM switching regulator controlling circuit, the logical circuit includes: an S/R latch circuit; a first Exclusive-OR circuit for receiving as its input the signal Vcomp and the signal PULSE-MIN to output a signal; and a second Exclusive-OR circuit for receiving as its input the signal Vcomp and the signal PULSE-MAX to output a signal. In addition, the logical circuit further includes: a second AND circuit for receiving as its input the output signal from the first Exclusive-OR circuit and the output signal from the second Exclusive-OR circuit to output a signal; a third AND circuit for receiving as its output the output signal from the second AND circuit and the output signal from the S/R latch circuit to output a signal Fup to a reset terminal of the S/R latch circuit; and a buffer circuit for receiving the signal Vcomp to output a signal EXT used to control an external switch. Further, the first AND circuit, in response to the output signal from the first Exclusive-OR circuit and the signal PULSE-MIN, outputs a signal Fdown to a set terminal of the S/R latch circuit, and the output signal from the S/R latch circuit is inputted to the oscillation circuit.
The PWM switching regulator controlling circuit according to the invention of the present application includes: an oscillation circuit for generating an oscillation signal for a PWM switching regulator; a comparator for receiving as its input the oscillation signal from the oscillation circuit and an output signal obtained by comparing a voltage signal obtained through division of a voltage Vout with a reference voltage of a reference voltage circuit in an error amplifying circuit to output a signal Vcomp used to control an external switch signal; and a logical circuit for receiving as its input the signal Vcomp to output a signal used to control an oscillation frequency of the oscillation circuit.
Further, in the PWM switching regulator controlling circuit, the logical circuit includes: an S/R latch circuit; a first time integrating circuit for receiving as its input the signal Vcomp to integrate a period of time during which an external switch signal is in an OFF-state to output a signal Fdown; and a second time integrating circuit for receiving as its input the signal Vcomp to integrate a period of time during which the external switch signal is an ON-state to output a signal Fup, and the output signal from the first time integrating circuit is inputted to a set terminal of the S/R latch circuit, the output signal from the second time integrating circuit is inputted to a reset terminal of the S/R latch circuit, and the output signal from the S/R latch circuit is inputted to the oscillation circuit.
In the PWM switching regulator controlling circuit according to the invention of the present application, when load condition is changed from a heavy load to a light load, an internal oscillation frequency is adapted to be changed from a first oscillation frequency to a second oscillation frequency lower than the first oscillation frequency, while when the load condition is changed from the light load to the heavy load, the internal oscillation frequency is adapted to be returned from the second oscillation frequency back to the first oscillation frequency in accordance with an ON/OFF ratio of a signal used to control a switch of a PWM switching regulator circuit.
In the PWM switching regulator controlling circuit according to the invention of the present application, a switch adapted to be switched in accordance with a light load and a heavy load is provided in a PWM switching regulator circuit, and when load condition is changed from the heavy load to the light load, an internal oscillation frequency is adapted to be changed from a first oscillation frequency to a second oscillation frequency lower than the first oscillation frequency by switching the switch, while when load condition is changed from the light load to the heavy load, the internal oscillation frequency is adapted to be returned from the second oscillation frequency back to the first oscillation frequency by switching the switch.
A PWM switching regulator according to the invention of the present application includes the PWM switching regulator controlling circuit according to a first or third aspect of the present invention.
An electronic apparatus according to the invention of the present application includes the PWM switching regulator.
An ON/OFF ratio (duty ratio) of a signal used to control an external switch of the PWM switching regulator is changed in accordance with load condition. Thus, an internal oscillation frequency is changed into a known oscillation frequency when the switching regulator is operated under a light load in accordance with the ON/OFF ratio.